1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera.
2. Description of the Related Art
In a zoom lens system for a compact camera, in addition to a requirement of an increased zoom ratio, a shorter focal length at the short focal length extremity has been required in recent years. In a zoom lens system for a single lens reflex (SLR) camera, a retrofocus zoom lens system including a negative front lens group and a positive rear lens group, in this order from the object, has been employed, since a long back focal distance is required to secure a space for providing a mirror. Unlike a SLR camera, a compact camera does not require a long back focal distance, so that a telephoto zoom lens system including a positive front lens group and a negative rear lens group, in this order from the object, is suitable for a zoom lens system thereof.
For compact cameras, conventional zoom lens systems with a zoom ratio of about 3 have employed a three-lens-group arrangement. The main reason for this tendency is that the correcting of field curvature over the entire zooming range is difficult if an attempt is made to achieve a high zoom ratio in a two-lens-group arrangement. In contrast to the two-lens-group arrangement, with the three-lens-group arrangement, by varying the distance between the first lens group and the second lens group, the correcting of field curvature can easily be made even if a high zoom ratio is set.
However, if an attempt is further made to obtain a zoom ration of about 4, the change in the distance between the first and second lens groups becomes longer, thereby the overall length of the zoom lens system becomes long. Furthermore, if the half angle of view at the short focal length extremity exceeds 35xc2x0, since the diameters of the first and second lens groups are increased in order to collect peripheral illumination, the size of the entire zoom lens system is inevitably increased, which results in an increase of both camera thickness (front-to-back length) and height (up-to-bottom length) for accommodating the zoom lens system.
It is an object of the present invention to provide a small three-lens-group zoom lens system of telephoto type which has a zoom ratio of about 4.0, and has the half angle of view at the short focal length extremity is more than 35xc2x0.
According to the present invention, the following effects (i) to (iii) can be attained by determining the optical power (hereinafter, power) of the third lens group, and the combined power of the first and second lens groups so that a sufficient space is secured between the first and second lens groups to move, while the overall length and diameter of the lens system can be reduced:
(i) a high zoom ratio, e.g., about 4;
(ii) a shorter focal length at the short focal length extremity, e.g., the half angle of view thereat more than 35xc2x0; and
(iii) miniaturization of the lens system.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object. zooming is performed by moving the first through third lens groups in the optical axis direction. The zoom lens system satisfies the following conditions:
0.62 less than (dT12xe2x88x92W12)/fW less than 1.0xe2x80x83xe2x80x83(1)
4 less than fT/f12T less than 6xe2x80x83xe2x80x83(2)
5 less than fT/|f3G| less than 8xe2x80x83xe2x80x83(3)
wherein
dT12 designates the distance between the first lens group and the second lens group at the long focal length extremity;
dW12 designates the distance between the first lens group and the second lens group at the short focal length extremity;
fW designates the focal length of the entire zoom lens system at the short focal length extremity;
fT designates the focal length of the entire zoom lens system at the long focal length extremity;
f12T designates the combined focal length of the first lens group and the second lens group at the long focal length extremity; and
f3G designates the focal length of the negative third lens group.
The zoom lens system preferably satisfies the following condition:
0.7 less than dT12/fW21 1.0xe2x80x83xe2x80x83(4)
The most-object side surface of the positive first lens group can be a concave surface facing towards the object, and the most-object side concave surface preferably satisfies the following condition:
xe2x88x925 less than r1/fW less than xe2x88x921xe2x80x83xe2x80x83(5)
wherein
r1 designates the radius of curvature of the most-object side concave surface of the first lens group.
The most-object side lens element of the positive first lens group can be a negative lens element, and this negative lens element preferably satisfies the following condition:
xe2x88x920.7 less than f1P/fW less than xe2x88x920.1xe2x80x83xe2x80x83(6)
wherein
f1P designates the focal length of the most-object side negative lens element of the first lens group.
The positive first lens group can include a negative first lens element and a positive second lens element, in this order from the object.
In the positive second lens group can include a lens element having at least one aspherical surface, and the aspherical surface preferably satisfies the following condition:
xe2x88x921000 less than xcex94IASP less than xe2x88x92500xe2x80x83xe2x80x83(7)
wherein
xcex94IASP designates the amount of change of the spherical aberration coefficient due to the aspherical surface under the condition that the focal length of the entire zoom lens system at the long focal length extremity is converted to 1.0.
In the negative third lens group can include a lens element having at least one aspherical surface, and the aspherical surface preferably satisfies the following condition:
0 less than xcex94VASP less than 0.2xe2x80x83xe2x80x83(8)
wherein
xcex94VASP designates the amount of change of the distortion coefficient due to the aspherical surface under the condition that the focal length of the entire zoom lens system at the short focal length extremity is converted to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-019917 (filed on Jan. 28, 2000) which is expressly incorporated herein by reference in its entirety.